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First detection of gas molecules in GRB galaxy

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 07 January, 2009

Using a combination of space- and ground-based telescopes, astronomers have, for the first time, identified molecular gas in the host galaxy of gamma-ray burst 080607.

The gamma-ray burst (GRB) was detected by the Swift Space Telescope in June last year, and the findings, which include follow-up observations from the W. M. Keck Observatory in Hawaii, provide insight into star formation when the Universe was just 2.2 billion years old.

"This burst gave us the opportunity to 'taste' the star-forming gas in a young galaxy more than 11 billion light-years away," says University of California professor Xavier Prochaska. The NASA-supported Peters Automated Infrared Imaging Telescope (PAIRITEL) on Mount Hopkins, Arizona, and the Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory on Mount Hamilton, California were also scrambled to observe the burst's afterglow.

GRBs are the Universe's most luminous explosions, triggered when a massive star runs out of nuclear fuel. As the star's core collapses into a black hole or neutron star, gas jets punch through the star’s expelled layers and out into space. Bright afterglows occur as the jets heat gas that was previously shed by the star. By taking spectra of the afterglow, astronomers gleaned information regarding the composition of the dust, gas and metals in the interstellar medium through which the light passed on its way out of the host galaxy and towards the Earth. In addition to the first clear detection of molecular gases – both carbon monoxide and hydrogen – the spectrum indicated a metal composition (elements heavier than hydrogen and helium) comparable to that of the Sun.

"We clearly see absorption from two molecular gases: hydrogen and carbon monoxide,” says Prochaska. “Those are gases we associate with star-forming regions in our own Galaxy”.

The Peters Automated Infrared Imaging Telescope (PAIRITEL) in Arizona caught GRB 080607’s afterglow (circled) about three minutes after the explosion. The afterglow’s light has been greatly dimmed and reddened by interstellar dust in its host galaxy, 11.5 billion light years away. Image: Adam Miller and Daniel Perley/UC Berkeley.

Astronomers have long expected to find evidence of these molecular clouds in other GRB host galaxies, but until now, efforts have been unsuccessful. The new observations indicate that star formation in the early Universe occurred in environments similar to star-forming regions in the Milky Way. Furthermore, the team believes that the burst exploded behind a molecular cloud so dense that less than one percent of the afterglow’s light was able to penetrate it.

"Intrinsically, this afterglow is the second brightest ever seen.
That's the only reason we were able to observe it at all," says Prochaska. The spectrum from Keck established that the explosion took place 11.5 billion light-years away, when the Universe was just 2.2 billion years old. However, nearly half of the absorption lines found in the Keck spectrum are currently unidentified.

"This is easily the most fascinating spectrum that I've ever worked on," says Prochaska. "Nearly half of the features remain a mystery, and it is possible that no one has ever detected them previously, either in controlled laboratory experiments or in spectra from our Galaxy or other galaxies." The team expects that understanding these mystery spectra will provide new data on the simplest space molecules.

Prochaska adds that there is also more hydrogen in this spectrum than along any path through the Milky Way. "This remains a bit of a puzzle. For now, we don't know much about the galaxy that hosted the explosion, but the evidence suggests it has been prodigious in terms of star formation."

Later this month the researchers will image the galaxy to connect their findings on the star-forming region with its global properties.

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